Introduction to Polarisation of Light
Definition: The property of light waves that describes the orientation of the oscillations in the transverse plane is known as polarisation.
Types of waves:
Transverse waves
Longitudinal waves
Polarisation in Transverse Light Waves
In transverse light waves, the electric field oscillates perpendicular to the direction of propagation.
The direction of the electric field vector determines the polarisation of the light wave.
When the electric field oscillates in a specific direction, it is said to be polarised in that direction.
Polarisation by Selective Absorption
When unpolarised light passes through a polaroid sheet, it gets polarised.
Polaroid sheet: A sheet that is made of a material which selectively absorbs light waves vibrating in certain directions
Example: Polaroid sunglasses
Polarisation by Reflection
When light reflects at an angle of incidence equal to the polarising angle, the reflected light becomes completely polarised.
The polarising angle is the angle of incidence at which the reflected and refracted rays make a right angle with each other.
The reflected light is polarised parallel to the reflecting surface.
Brewster’s Law
Brewster’s law states that the tangent of the polarising angle is equal to the refractive index of the medium.
Mathematically, n = tanθp, where n is the refractive index and θp is the polarising angle.
The polarising angle depends on the nature of the medium through which light is incident.
Polarisation by Scattering
When sunlight passes through the atmosphere, it undergoes scattering due to particles and molecules in the air.
This scattering process is responsible for the blue color of the sky, as blue light is scattered more than other colors due to its shorter wavelength.
When light scatters, it becomes partially polarised in the plane perpendicular to the direction of propagation.
Circular Polarisation
Circular polarisation occurs when the electric field vector of a light wave rotates in a circle as the wave propagates.
Circularly polarised light can be either left-handed or right-handed, depending on the direction of rotation.
Circular polarisation is produced by passing linearly polarised light through a quarter-wave plate.
Malus’ Law
Malus’ law describes the intensity of polarised light after passing through an analyser.
It states that the intensity (I) of the transmitted light is proportional to the square of the cosine of the angle (θ) between the polariser and analyser:
Where I₀ is the maximum intensity when the polariser and analyser are aligned (θ = 0°).
Double Refraction
Double refraction occurs in certain transparent materials, such as calcite, when a light wave passes through them.
In double refraction, the incident light wave splits into two rays, known as the ordinary ray and the extraordinary ray.
The ordinary ray obeys the laws of refraction, while the extraordinary ray follows a different path due to the crystal structure.
Polarisation by Optical Activity
Certain substances, such as sugar solution and quartz, can rotate the plane of polarisation of light passing through them.
This phenomenon is known as optical activity.
The amount of rotation depends on the substance used and the thickness of the sample.
Optical activity is used in polarimeters to measure the concentration of optically active substances.
Polarisation of Light Waves
The polarisation of light waves is a fundamental property of electromagnetic waves.
It plays a crucial role in various optical phenomena, such as polaroid filters, reflections, scattering, and double refraction.
Understanding and manipulating the polarisation of light is essential in many applications, including telecommunications, photography, and display technologies.
LCD displays: Liquid Crystal Displays use the property of polarisation to control the intensity of light passing through different pixels.
3D movies: Polarised glasses are used in 3D movies to separate left-eye and right-eye images, creating a sense of depth perception.
Communication systems: Polarisation is used in optical fiber communication systems to transmit signals with high data rates.
Photography: Polarising filters are used in photography to reduce glare and enhance colors in certain conditions.
Material testing: Polarisation can be used to determine the internal stress and strain of materials.
Electromagnetic waves, including light waves, are transverse waves consisting of electric and magnetic fields that oscillate perpendicular to each other.
The electric field direction determines the polarisation of the wave.
Electromagnetic waves can have different polarisations: linear, circular, or elliptical.
Linear polarisation is the simplest form, where the electric field oscillates in a single direction.
When unpolarised light reflects off a non-metallic surface, it becomes partially polarised.
The reflected light is polarised parallel to the surface if the incident angle is greater than the polarising angle.
The polarising angle can be calculated using Brewster’s law: θp = arctan(n), where n is the refractive index of the medium.
Example: When light reflects off a water surface at an incident angle of 53°, the reflected light will be partially polarised with the electric field parallel to the surface.
Scattering of light by small particles or molecules in the atmosphere causes the sky to appear blue.
The scattered light becomes partially polarised, with the electric field oscillating in the plane perpendicular to the direction of propagation.
This polarisation of scattered light can be observed by using a polarising filter and looking at the sky at a 90° angle from the sun.
Example: On a clear day, the sky appears bluer when viewed through a polarising filter due to the selective polarisation of scattered light.
Light waves can be expressed as the sum of two linearly polarised waves with different amplitudes and phase differences.
The electric field of a linearly polarised wave can be written as E = E₀sin(ωt - kz).
Circularly polarised waves can be expressed as the sum or difference of two orthogonal linearly polarised waves.
Example: A circularly polarised wave can be written as E = E₀(sinωtcoskz + cosωtsinkz).
Polaroids are filters that only transmit light waves with a specific polarisation direction.
A polaroid consists of long-chain organic molecules aligned in a specific direction to act as a polariser.
Using two polaroids with their transmission axes perpendicular to each other, we can completely block the passage of light.
Example: When two crossed polaroids are used, no light will pass through since the polarisation directions are perpendicular.
Malus’ Law relates the intensity of transmitted light to the angle between the polariser and analyser.
It can be mathematically expressed as I = I₀cos²θ, where I is the transmitted intensity, I₀ is the initial intensity, and θ is the angle between the transmission axes of the polariser and analyser.
Analyzing polarised light involves determining the polarisation state and intensity of a light wave.
This can be done using polarisers, quarter-wave plates, and other optical components.
Analyzing polarised light is important in various applications, such as optical microscopy, spectroscopy, and laser technology.
Example: In optical microscopy, polarised light can be used to enhance contrast and highlight specific features in the sample.
Some substances have the ability to rotate the plane of polarisation of light passing through them.
This phenomenon is known as optical activity and is observed in certain organic and inorganic compounds.
The extent of rotation depends on the nature of the substance and the length of the path the light travels.
Example: Sugar solutions, such as glucose, exhibit optical activity and can be used to demonstrate the rotation of polarised light.
Circular dichroism is the differential absorption of left and right circularly polarised light by certain materials.
This property allows for the analysis of molecular and electronic structures in substances such as proteins and organic compounds.
Circular dichroism spectroscopy is a powerful technique in the field of biochemistry and structural biology.
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3D glasses use the principle of polarisation to create a three-dimensional effect.
Polarised glasses have different polarisation directions for each eye, allowing separate images to be seen by each eye.
The brain combines these separate images to create the perception of depth.
3D movies typically use circular polarisation, where the polarisation direction of the light is rotated in a circular pattern.
Circularly polarised glasses are more comfortable to wear and provide a better 3D experience.
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Polarisation is used in optical fiber communication systems to transmit signals with high data rates.
The fibre optic cables used in these systems have a specific polarisation direction for the transmitted light.
This polarisation direction is maintained throughout the length of the cable to ensure efficient transmission.
By using different polarisation directions for different signals, multiple channels can be transmitted simultaneously over a single fiber.
Polarisation mode dispersion (PMD) can occur in these systems and needs to be minimized to maintain signal quality.
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Polaroid sheets are commonly used in sunglasses and camera filters to reduce glare and improve visibility.
When light is reflected or scattered, its waves become partially polarised in specific directions.
Polaroid sheets are aligned to block light waves with these polarisations, reducing glare and enhancing visibility.
The sheets are also used in photography to enhance colors, reduce reflections, and improve image quality.
Example: Polaroid sunglasses reduce glare from water or shiny surfaces, making it easier to see and reducing eye strain.
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Liquid crystal displays (LCDs) utilize the property of polarisation to control the intensity and color of light passing through pixels.
LCD screens consist of a layer of liquid crystals sandwiched between two polarisers.
By applying electric fields to the liquid crystals, their orientation can be controlled, allowing light to pass through or block it.
The polarisers and liquid crystals work together to produce the desired colors and images on the screen.
Example: A deep black color on an LCD screen is achieved by blocking polarised light from the backlight.
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Polarisation techniques can be used to analyze the internal stress and strain of materials.
Photoelasticity is a method that utilizes the birefringent property of some materials.
By subjecting a stressed material to polarised light, the patterns of interference can reveal stress distribution.
This technique is used in material testing, engineering, and the study of mechanical properties.
Example: Studying the stress patterns in a transparent plastic part to determine the areas most susceptible to failure.
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Optical filters can selectively transmit or block light with specific polarisation directions.
Polarisation filters can be used to enhance contrast, reduce glare, or separate polarised light for analysis.
Examples of optic filters include polarising film, wave plates, dichroic filters, and crystal filters.
These filters are widely used in various industries, including photography, microscopy, and spectrometry.
Example: A polarising film on a camera lens can reduce reflections on water surfaces and increase color saturation.
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Polarised light microscopy is a valuable tool for the study of various materials, including minerals, crystals, and biological samples.
It allows the observation of birefringent properties, which provide valuable information about the structure and composition of materials.
Polarised light microscopy can be used for mineral identification, examining tissue samples, and analyzing defects in materials.
The technique relies on polarisers, analyzers, wave plates, and other components to control and analyze polarised light.
Example: Polarised light microscopy can reveal hidden structures in minerals, such as twinning or strain patterns.
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Spectroscopy techniques, such as circular dichroism and polarimetry, rely on polarising light to study molecular structures and interactions.
Circular dichroism measures the differential absorption of left and right circularly polarised light by chiral molecules.
Polarimetry measures the rotation of the plane of polarisation caused by optically active substances.
These techniques are widely used in fields such as biochemistry, pharmacology, and materials science.
Example: Circular dichroism can provide insights into the secondary structure and folding of proteins.
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Laser light is typically highly polarised, with the electric field oscillating in a single direction.
Polarisation control is essential in laser technology to optimize laser performance and beam quality.
Polarisation optics, such as polarisers and wave plates, are used to control and manipulate the polarisation of laser beams.
Certain laser cavity designs require specific polarisation states for efficient operation.
Example: A polarisation-maintaining fiber laser can produce a high-quality and stable laser beam with a specific polarisation state.
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The understanding and control of polarisation have led to advancements in various fields, such as telecommunications and imaging.
Future applications might include polarisation-based quantum communication and computing.
Polarisation sensing and imaging techniques can have applications in medical diagnostics and remote sensing.
Utilizing polarisation in augmented and virtual reality technologies might enhance user experiences.
The continued study and utilization of polarisation in different fields will likely lead to further advancements and innovative applications.